Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver salt photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities. Thus the images obtained are outstanding for reproducibility of halftones and gradation expression, and exceptionally high-definition images can be delivered.
Moreover, the dye diffusion transfer recording system has advantages that it is a dry process, it permits direct visualization from digital data, and it allows simple reproduction; and its market as a full-color hard copy system has been expanding.
As such a dye diffusion transfer recording system has become popular, improvements of printing speed have been progressing; and it has become apparent that sufficient color generation densities are no longer obtained by use of traditional ink sheets and image-receiving sheets and application of thermal energies utilized hitherto to those sheets. Therefore, improvements have been made by a method of applying greater thermal energy than ever at the time of printing, and a method of enhancing thermal conduction efficiency by reducing the thickness of the support of ink sheet. These actions for increasing thermal energy to be applied to an image-receiving sheet at the time of printing, necessitates an image-receiving sheet that can ensure to perform both responsiveness to low thermal energy in low-density portions and responsiveness to high thermal energy in high-density portions. In general, a thermoplastic polymer is used as a receptor polymer of an image-receiving sheet, and the compatibility between transferring property of dyes (generally, the lower Tg the thermoplastic resin has, the higher transferring property of dyes the resin has) and releasing property from ink sheet (the higher Tg the thermoplastic resin has, the less the resin causes sticking due to welding) is ensured by controlling the glass transition temperature (Tg) of the thermoplastic polymer. Therefore, it is difficult to achieve both transferring property of dyes and releasing property from ink sheet at the same time over a wide range of temperatures.
Further, a method of providing a protective layer on an image surface by lamination, for the purpose of protecting the image surface and enhancing image fastness, has become the mainstream of the recent dye diffusion transfer recording system. Therefore, it is required to give attention to transferring property of protective layer to a heat-sensitive transfer image-receiving sheet. In general, the temperature at the time of transferring a protective layer is adjusted to a temperature lower than dye transfer temperatures (at least lower than the temperature to achieve the maximum density), for the purpose of avoiding thermal diffusion of the image. Thus, an image-receiving sheet which has good releasing property from ink sheet tends to have difficulty in transferring thereon a protective layer from the ink sheet.
In order to solve these problems, methods of introducing a releasing agent to the image-receiving sheet surface have been proposed. For instance, Japanese Patent Nos. 2572769 and 2854319 describe addition of a releasing agent, such as polyethylene wax, amide wax, or Teflon (trademark) powder, to a receptor layer of a heat-sensitive transfer image-receiving sheet, and disclose inventions which improve releasing property from ink sheet. However, these patents are silent on a method for ensuring compatibilities with transferring property of dyes and transferring property of protective layer.
JP-A-11-321139 (“JP-A” means unexamined published Japanese patent application) discloses a method of introducing carnauba wax to a receptor layer made up of a certain polyester compound. Therein, it is described that the method can prevent sticking effectively and can improve the releasing property from ink sheet.
JP-A-2005-238748 discloses the method of enhancing releasing property from ink sheet while heightening the transfer densities, by introduction of urethane-modified wax to the image-receiving sheet. However, this reference is also silent on a method for ensuring the compatibility with transferring property of protective layer.
The image-receiving sheets disclosed in these patent documents established certain levels of compatibilities with transferring property of dyes and releasing property from ink sheet, and further with transferring property of protective layer which was achieved by addition amount adjustment. However, they have not reached sufficient levels yet, so they are not satisfactory. Further, any methods for positively enhance the transferring property of protective layer are not suggested in these published patents.